The electric dust precipitator, in principle, comprises a pair of opposite dust collecting electrodes and a discharging wire disposed intermediately therebetween, which are positive and negative electrodes, respectively. When a high direct current voltage is applied between the electrodes and the wire, the electric field generated near the discharging wire is distorted so that a negative corona discharge may take place.
When a gas to be treated, for example, a waste gas containing smoke dust is fed through the space between the dust collecting electrodes and the discharging wire, i.e., through the discharging region, dust particles in the gas are negatively charged and then attracted to the dust collecting electrodes.
The method for removing fine particles in the gas in accordance with the above-described principle is known to be highly effective for dust precipitation.
All the prior electric dust precipitators depend on the above principle, though modifications and variations are, of course, made in practice. One of the improvements is made in accordance with the removal of the dust precipitated on dust collecting electrodes. Usually, the collected dust is removed in a mechanical manner. In one manner, dust collecting electrodes are shocked or vibrated by means of a hammer to knock off the collected dust. In an alternate manner, a slider or scraper is slidably moved along the surface of dust collecting electrodes to scrape off the dust.
The former method, however, has serious drawbacks in that first of all, a shock wave caused by the hammer is so violent that beams supporting the electrodes tend to fail. Particularly, if the dust collecting electrode is of a tubular type through which cooling water is passed, failure or cracks of the electrodes or joints between the electrodes and the beams (or headers) may result in leakage of water. Besides, the shock given by the hammer is partly absorbed by the cooling water within the tubular electrodes resulting in insufficient vibration of the electrodes. Further, it is difficult to effectively knock off highly adhesive dust even at more accelerated vibrations of the hammer. Such insufficient removal of dust necessitates periodic suspension of operation to clear the electrodes.
In addition, the dust which has adhered to the electrodes instantaneously drops immediately after the hammer makes impact with the electrodes. Upon settling to the bottom, the dust disperses again and as a result, dust in the discharging space between the dust collecting electrodes instantaneously increases to a concentration several tens to several hundreds times higher than usual. The corona discharge in the relevant space is suppressed by such higher concentrations of dust so that the dust collecting capacity is reduced to a large extent and the concentration of dust at the exit of the precipitator is temporarily increased.
The latter method, removal of dust by means of a sliding scraper, can overcome the above-described drawbacks inherent to the hammering method while itself having a disadvantage. The prior sliding scraper is moved up and down in a sliding relation with the surface of the dust collecting electrodes. Dust scraping, however, is still not complete. Particularly, when the sliding scraper is to be moved during operation dust precipitates on the back of the scraper itself, too and the operation must be suspended to remove the dust precipitated on the back.
Another prior improvement is made on the structure of electrodes. Since the resistivity of dust which varies depending on the surrounding temperature is generally high at elevated temperatures, back ionization with sparking may often occur in the vicinity of the dust collecting electrodes, eventually decreasing the voltage applied between the electrodes and the wires. The dust collecting capacity is then reduced. To avoid such undesirable phenomenon, there is proposed a dust collecting electrode is proposed, of a hollow plate-type, through which a cooling fluid is passed to control the temperature of the electrode. The temperature of the dust in the vicinity of the cooled electrode is also decreased, resulting in a lower resistivity sufficient to prevent back ionization. British Pat. No. 643,363, is herein incorporated by reference.
Such proposals, however, deal with precipitators of a very small type used for air-conditioners or in experimental stage. The structure of electrodes which can be used on an industrial scale has never been proposed. By way of illustration, reference is made to an industrial electric dust precipitator of an electrode-cooling type. Usually, dust collecting electrodes in pairs are spaced apart from each other at a distance of 200 to 250 mm (the distance between opposite electrodes means the distance between the centers of opposite electrodes). Since the dust collecting electrode is hollow and cooling water can be passed therethrough, the wall of the electrode must be thick, for example 20 to 50 mm in thickness. The effective dust collecting space is then reduced by about 10 to 25% by volume. The rate of gas flow in the precipitator increases in proportion to the above reduction and hence the charging time is shortened and the lesser the charging time, the lesser the dust collection.
Since the distance between the opposite dust collecting electrodes is small, for example 200 to 250 mm, the number of electrodes set within a unit volume is increased and the dust collecting area and hence the heat transfer area are large. This in turn requires a larger amount of cooling water. If the amount of cooling water is reduced below a certain level, the difference in the temperature of the cooling water at the entrance and the exit is considerably larger. When the temperature of the cooling water at the entrance is maintained above the dew point of the gas to prevent the corrosion of the electrodes, the upper portions of the electrodes which are near the exit of the water and hence the dust on the upper portions are cooled insufficiently. Therefore, the dust on the upper portions is hardly cooled below a critical level for back ionization. Particularly, when the distance between the dust collecting electrodes is within 200 to 250 mm, ionic wind flows at higher velocities along the electrodes, the temperature of dust on the surface of the electrodes increases and consequently, it becomes difficult to extinguish back ionization under these conditions. Further, the ionic wind at higher velocities makes a boundary film on the surface of the electrodes thinner, increasing the quantity of heat required for cooling. This adds to the above-described increase of heat transfer area creating further multiplied adverse effects to the system. To overcome such influences, it is required to substantially increase the capacities of a pump, a cooler and other equipment included in a system for circulating the cooling water. As a result, not only the costs for installation and operation of the precipitator are too high, but also the efficiency of dust collection is adversely affected. These problems in the foregoing, mainly comprise a hindrance to the application of the industrial electric dust precipitator as commercially available equipment.
The prior means for controlling the temperature of electrodes, for example, in the case of a cooling system, deals with only the temperature of the cooling water. Since back ionization has a variable relation with the temperature, such means cannot follow such variations, resulting in incomplete prevention of back ionization.
Further, the prior dust collecting electrode through which a cooling fluid is passed is of a hollow plate type. This structure includes a number of welded portions where mechanical strain readily takes place. It is then difficult to manufacture electrodes of medium and large scale. Besides, the hollow plate electrode has a flat surface and this surface area is large and consequently, dust is easily re-dispersed so that the corona discharge may often be suppressed.
Therefore, it is a primary object of this invention to obviate the above-described drawbacks and to provide an improved electric dust precipitator.
Essentially, according to the present invention there is provided an electric dust precipitator comprising a chamber, a pair of dust collecting electrode groups disposed in parallel with each other and with the direction of gas flow in said chamber and spaced apart by a distance of more than 400 mm, each electrode being made of a tubular member, except for a hollow plate type, through which a cooling fluid for controlling the temperature thereof is passed, and a plurality of discharging wires disposed in parallel with said groups and between the dust collecting electrode groups, wherein a high direct current voltage is applied between the discharging wires and the dust collecting electrodes, and a control system including a detecting means which detects the number and quantity of spark discharges caused from back ionization per unit time and a control means which controls the temperature of said cooling fluid with the aid of a detected signal.